When microelectronic devices are produced on a semiconductor wafer, it is crucial that the critical dimensions of the devices be held within specified tolerances. Critical dimensions, in this context, refer to the widths of features, such as conductors, that are deposited on the wafer and the spacing between adjacent features. Deviations from the specified dimensions lead to performance and yield degradation. Most microelectronic production facilities currently use optical metrology to monitor critical dimensions. As semiconductor devices become ever denser, however, with design rules of 0.25 μm and below, it is growing increasingly difficult for classical optical metrology systems to provide sufficiently accurate results.
X-ray measurement of critical dimensions has been proposed as an alternative to optical measurement. Systems and methods for this purpose are described, for example, in U.S. Pat. No. 6,556,652, whose disclosure is incorporated herein by reference. According to the method described in this patent, a surface of a substrate is irradiated with a beam of X-rays. A pattern of the X-rays scattered from the surface due to features formed on the surface is then detected and analyzed to measure a dimension of the features in a direction parallel to the surface.
Another method for X-ray based CD measurements is described by Jones et al., in “Small Angle X-ray Scattering for Sub-100 nm Pattern Characterization,” Applied Physics Letters 83:19 (2003), pages 4059-4061, which is incorporated herein by reference. The authors use transmission-mode small angle X-ray scattering (SAXS) with a synchrotron X-ray source to characterize a series of polymer photoresist gratings formed on a substrate. The X-ray beam passes through the grating and the substrate, and the SAXS pattern is measured using a two-dimensional CCD detector. The photoresist grating produces a one-dimensional series of diffraction spots in the SAXS pattern on the detector. The SAXS intensity as a function of the scattering vector q is analyzed to determine the grating spacing and sidewall angle.
Alternatively, X-ray based CD measurements may be carried out in reflection mode, as described, for example, in U.S. Pat. No. 7,110,491, whose disclosure is incorporated herein by reference. A beam of X-rays is directed to impinge on the area of a periodic feature on a sample, and the X-rays scattered from the surface are detected as a function of azimuth. The scattered X-rays exhibit a pattern of diffraction, which is analyzed in order to extract information regarding the dimensions of the periodic feature.